1. Field of the Invention
The present invention relates to offshore platforms and, more particularly, is concerned with floating drilling and production platforms which are used in the exploration and production of offshore oil and gas.
2. Description of the Prior Art
Drilling/production operations in exploration and production of offshore oil and gas require a floating platform that is as stable as possible even in very severe environments. Among the six degrees of freedom of a floating platform, the heave, pitch and roll motions, usually referred to as vertical motions, are particularly important for drilling production operations. For platforms with small vertical motions, rigid, high-pressure risers can be used to connect the platform to the wellhead on the sea floor. With rigid, high-pressure risers, the wellhead is virtually moved from the sea floor to the platform. Therefore, wellhead equipment, such as blowout preventors and control valves, can be installed on the platform. For platforms with large vertical motions, flexible, low-pressure risers must be used, and wellhead equipment must be installed on the sea floor. Equipment placed on the sea floor is much more difficult and costly to manufacture, install, and maintain. In the past, various designs of floating platforms have been developed in the offshore industry. Following is a brief description of the floating platforms which have been designed or suggested for oil gas field development:
1) Semi-submersible Floating Production System (FPS)
A semi-submersible FPS is made up of a number of columns and pontoons. A conventional chain-wire catenary spread mooring system is used for station keeping. FIG. 1a illustrates a typical semi-submersible FPS. The first semi-submersible FPS was installed in 1975 in the central North Sea. The vertical motions of a semi-submersible FPS are too large for rigid, high-pressure risers.
2) FPSO (Floating Production, Storage and Offloading
An FPSO is a monohull, generally ship-shaped, production system that rotates around an integral mooring turret so that the bow generally points to the prevailing seas. Environmental loads and vessel response are thereby reduced. FIG. 1b illustrates a typical FPSO. The earliest FPSO was employed in the West Africa/Mediterranean region in 1977. The main advantage of an FPSO is that it provides large oil storage and offloading capacity, eliminating the need for a pipeline. However, an FPSO also suffers from large vertical motions and high cost for the mooring turret. In addition, it has technology limitation in fluid swivel for high-pressure gas export.
3) TLP (Tension Leg Platform
A TLP is a semi-submersible-like platform which is pulled down to a deeper submergence from its equilibrium position by a number of vertical tensioned legs. FIG. 1c illustrates a typical TLP. The first TLP was installed in the North Sea Hutton Field in 1985. The vertical motions of a TLP are virtually restrained by its tension legs. The tension legs are typically steel tubes that are 28 to 32 inches in diameter and xc2xe to 3 inches thick. A TLP can accommodate rigid, high-pressure risers. A disadvantage of a TLP is its high cost due to the fabrication installation of tension legs and subsea piles that are used to secure the tension legs to the sea floor, especially when applied to deep water. Another disadvantage is the additional vessel displacement (about 20% of system weight) required to maintain the tension in the tension legs.
4) Conventional Spar
A spar is a vertical circular cylinder with a small water plane, usually about 30 m in diameter, and a deep draft, usually around 200 m. A conventional chain-wire is used for station keeping. FIG. 1d illustrates a typical conventional spar. The world""s first spar was installed in Gulf of Mexico in 1996. The deep draft of a spar is necessary to maintain its stability and to help reduce the vertical wave exciting force acting on it. A spar has very small vertical motions and can accommodate rigid risers. The disadvantages of a spar include high construction and transportation cost due to its large size, high installation cost due to offshore topside-hull mating, limited deck size, and limited well bay space. Moreover, because environmental forces and mooring force act on different part of the long hull, the heel angle of a spar can be quite large, which have a negative impact on riser design.
5) Truss Spar
A truss spar is similar to a conventional spar in structure except that the lower portion of a truss spar is a truss structure, not a circular shell structure. FIG. 1e illustrates a typical truss spar. A truss spar uses heave plates to reduce its heave motion. The truss is used to lower the ballast weight to keep the center of gravity lower than the center of buoyancy. Although its construction cost is expected to be a little lower than that of a conventional spar, a truss spar still suffers from the same disadvantages a conventional spar has. The truss spar concept has not yet been utilized in any oil/gas field development.
The major disadvantages of the above described platforms can be summarized as follows:
(a) Large vertical motions: semi-submersible and FPSO;
(b) Large structure size: spar hull;
(c) Expensive tension legs: TLP;
(d) Difficult offshore deck-hull mating: Spar;
(e) Limited deck area: conventional and truss spar;
(f) Limited well bay space for risers: conventional and truss spar.
Accordingly, several objects and advantages of our invention are:
(a) to provide a floating platform which has small vertical motions,
(b) to provide a floating platform which is much shorter than a conventional or truss spar,
(c) to provide a floating platform which does not require tension legs to restrain its vertical motions,
(d) to provide a floating platform which does not require offshore deck-hull mating,
(e) to provide a floating platform which has large deck area, and
(f) to provide a floating platform which has large space to accommodate risers.
The present invention provides a heave suppressed offshore drilling and production platform that provides the above listed advantages. According to a first aspect of the invention, there is provided a floating offshore drilling and production platform having a deck for drilling and production operations and a plurality of buoyant columns extending from the lower side of the deck, at least some of the columns having vertical shafts therein, the shafts being open on their lower ends. For each column having a vertical shaft therein, a vertically extending truss leg loosely is fitted within the shaft, wherein the truss leg may be lowered axially out the open lower end of the shaft. A plate is attached to the lower ends of the truss legs for suppressing heave and vertical motions of the floating platform when lowered by the truss legs to a position substantially below the water surface.
According to a second aspect of the invention, there is provided a method for installing, at a deep water offshore drilling and production site, a heave suppressed, floating platform. The platform has a deck and buoyant columns. The columns have open shafts therein. The method comprises launching the platform offshore into relatively shallow water; floating the platform to the deep water drilling and production site; and lowering a plate attached to a plurality of truss legs to a position substantially below the bottom of the platform by extending the truss legs out the open shafts of the columns, so that the hydrodynamic mass of the plate suppresses heave and vertical motions of the platform.
According to a third aspect of the invention, there is provided a floating offshore drilling and production platform. The platform includes a deck for drilling and production operations. A plurality of buoyant columns extend from the lower side of the deck. Means is provided for connecting adjacent columns near their lower ends. At least one vertically movable truss is loosely fitted to the columns, wherein the at least one movable truss may be lowered with respect to the columns. Means is attached to the bottom of the at least one movable truss for suppressing heave and vertical motions of the platform when said means is lowered by the movable truss to a position substantially below the water surface.
According to a fourth aspect of the invention, there is provided a method for installing, at a deep water offshore drilling and production site, a heave suppressed, floating platform. The platform has a deck and buoyant columns extending from the lower side of the deck. The method comprises launching the platform offshore into relatively shallow water, floating the platform to the deep water drilling and production site, flooding an open barge affixed to the bottom of a vertically movable truss loosely fitted to the columns for further sinking the lower portion of the platform so that the deck is at the desired elevation above the sea surface, and lowering the open barge to a position substantially below the bottom of the platform by lowering the movable truss with respect to the platform so that the hydrodynamic mass of the open barge suppresses heave and vertical motions of the platform.
According to a fifth aspect of the invention, there is provided a method for installing, at a deep water offshore drilling and production site, a heave suppressed, floating platform. The platform has a deck and buoyant columns extending from the lower side of the deck. The method comprises launching the platform offshore into relatively shallow water; floating the platform to the deep water drilling and production site; and releasing air from an inverted barge affixed to the bottom of a vertically movable truss loosely fitted to the columns for further sinking the lower portion of the platform so that the deck is at the desired elevation above the sea surface. The method further comprises lowering the inverted barge to a position substantially below the bottom of the platform by lowering the movable truss with respect to the platform so that the hydrodynamic mass of the inverted barge suppresses heave and vertical motions of the platform.
According to a sixth aspect of the invention, there is provided a method for installing, at a deep water offshore drilling and production site, a heave suppressed, floating platform. The platform has a deck and columns extending from the lower side of the deck. The method comprises launching the platform offshore into relatively shallow water; floating the platform to the deep water drilling and production site; and flooding a heave pontoon affixed to the bottom of a vertically movable truss loosely fitted to the columns for lowering the heave pontoon to a position substantially below the bottom of the platform, so that the hydrodynamic mass of the flooded heave pontoon suppresses heave and vertical motions of the platform.
According to a seventh aspect of the invention, there is provided a method for installing, at a deep water offshore drilling and production site, a heave suppressed, floating platform. The platform has a deck and columns extending from the lower side of the deck. The method comprises placing a heave pontoon affixed to the bottom of a truss at a convenient place on the sea floor; launching the platform offshore; and floating the platform to a position above the heave pontoon and truss on the sea floor. The method further comprises lifting the truss and heave pontoon from the sea floor to the platform, so that the upper portion of the truss adjoins the lower portion of the platform; and securing the upper portion of the truss to the platform, whereby the heave pontoon suppresses heave and vertical motions of the platform.